Expose shallow handlers

[Willenbrink]

Currently this ppx exclusively uses deep handlers but shallow handlers will likely become the preferred approach (see the multicore newsletter, 'Programming with effect handlers').

I believe this ppx should either use shallow handlers exclusively (a breaking change) or allow using both by introducing new syntax, e.g. [%deep? ...]/[%shallow? ...] to follow upstream.

Note that the preferred handler type is not yet certain, from the newsletter above: "If shallow handlers becomes more widely in the coming years [...]". So this issue might be a bit premature.